Thermal transfer printing involves the controlled transfer of an ink (e.g., a colorant dispersed in a wax base material) from a carrier such as a polymer ribbon onto a print medium surface. A thermal transfer printer having a print head with a large number of independently activatable heating elements per unit of length is one prior art apparatus employed for this purpose. The ink/carrier structure is placed within the printer such that the carrier side is adjacent to the heating elements and the ink side is adjacent to a print media support upon which the print medium rests during printing.
To print an image, the print head contacts the print medium and ink is transferred to particular locations on the print medium surface when predetermined combinations of heating elements are activated adjacent to those image-forming locations. The ink/carrier structure is locally heated by the heating elements to a temperature at or above the melting point of the ink. In this manner, an amount of ink softens and adheres to the print medium at the predetermined locations to form the image. It is well known that the thermally transferred ink adheres better and more completely to a smooth print medium surface than to a rough surface such as that commonly found in plain paper. Therefore, specially coated paper is typically used in thermal transfer printers to improve the image quality and durability of printed images.
Color images are printed with an ink/carrier structure that includes separate regions of differently colored inks such as the subtractive primary colors, yellow, magenta, and cyan. Color printing is accomplished by sequential passes of the print medium past the print head, each pass selectively transferring different colored inks at predetermined times. Such thermally transferred color images transmit light rectilinearly and are therefore well suited to overhead transparency projection applications. Moreover, transparency film typically has a smooth surface to which thermally transferred ink readily adheres.
Because specially coated print media are relatively costly and not as commonly available as plain paper, some means is desired to improve the durability and image quality of thermal transfer images printed on plain paper. U.S. Pat. No. 4,704,615 of Tanaka for THERMAL TRANSFER PRINTING APPARATUS describes an ink/carrier ribbon having yellow, magenta, and cyan ink panels and an additional panel of pre-coat material (hereafter "pre-coat"). Pre-coat is thermally transferred to predetermined areas of a rough print medium, such as plain paper, that subsequently receive a thermally transferred image. Pre-coat adheres to rough media and provides a smooth, transparent, thermal adhesive base to which thermally transferred inks readily adhere. Alternatively, pre-coat can be white to improve the chroma and brightness of images printed on opaque or colored media.
Pre-coat has disadvantages, however, such as degradation of the light transmission through transparency film, and degradation of image quality, by forming a "halo" around images when applied to coated media. Colors may also be perceived differently when viewed under the various above-described combinations of media type and pre-coating.
Thermal printing ribbons are available with a single black panel, three color panels (yellow, magenta, and cyan), or four color panels, (yellow, magenta, cyan, and black). Such ribbons are typically supplied on spools that have encoded end caps to communicate to the printer whether the ribbon includes one, three, or four panels. All colored ribbons also have an encoding stripe that runs along one edge of the length of the ribbon to communicate panel location and color data to the printer.
Printers have been developed that can print images on various print media types including, but not limited to, plain paper, coated paper, and transparency film. Such print media types are also available in a variety of standard sizes such as letter (A-size) and international A4-size.
Some printers, such as one embodying this invention, clamp an edge of a selected print medium to a drum and wrap the print medium around the drum during printing. Such an arrangement provides accurate control of print medium positioning relative to the print head and is particularly advantageous in applications such as multi-pass color printing. However, because printing can not be accomplished adjacent to the clamped edge of the medium, special extended-length media are available that include a perforated tear-off strip. Such media can be clamped at the extended length portion and printed over a remaining standard length portion. After printing, the extended-length portion is removed at the perforation. Plain paper, coated paper, and transparency film are currently available in A-size and A4-size, with or without a perforated extended-length portion.
Because a wide variety of print media exists, printers have been developed that have multiple media feeders into which a variety of media trays can be fitted. The trays are each sized to hold specific sizes of paper and are encoded to communicate the paper size to the printer. Some trays are also fitted with a switch by which the media type loaded in the tray, such as "transparency" or "paper," can be selected and communicated to the printer.
Most printers can detect when the print medium in a tray has been depleted or the tray is not inserted and take appropriate actions such as halting printing, lighting an "out of paper" indicator, switching trays, and sending an "add paper" or "insert tray" message to the printer user. Because there is such a variety of media types and sizes, a thermal transfer printer capable of automatically determining whether to pre-coat user selected combinations of trays, media types, and media sizes would be advantageous.
Many printers include a control software driver program (hereafter "printer driver") for handling many of the variables described above. Such printer drivers are often interfaced to a computer programming language known as PostScript.RTM., which is available from Adobe Systems Inc., Mountain View, Cal. The PostScript.RTM. language, described in the PostScript.RTM. Language Reference Manual, Second Edition, 1990, Addison-Wesley Publishing Co., Reading, Mass., includes methods for manipulating text and graphics, selecting media sizes, types, trays, and the number of copies to be printed. These and other variables are collected together with data to be printed into a data file referred to as a print job. PostScript.RTM. also supports multiple print job queuing, sorts print job priorities, and handles errors detected in the print jobs.
Print job errors are ordinarily resolved by PostScript.RTM.. For example, if a requested paper size is depleted PostScript.RTM. may automatically switch (if multi-tray printer) and print on any available media in another tray. The problem is that PostScript.RTM. was developed without a foreknowledge of evolving technologies such as thermal transfer printing. New printing technologies require circumvention by the printer driver of some existing error resolution methods.
For example, in a thermal transfer printer, switching trays when a tray becomes empty does not address whether the alternate tray has coated paper, whether pre-coat ribbon is installed, or whether a tray has extended-length perforated paper (that may be either plain or coated). Moreover, the perceived colors of a thermal transfer image are altered by various combinations of media type and pre-coat. Acceptable image colors printed by one type of printer may not be acceptable colors when printed by a thermal transfer printer with arbitrary combinations of media type and pre-coat. PostScript.RTM. supports a post-rendering lookup table for correcting colors, but no method exists for switching color correction tables in response to media type and pre-coat combinations found in multi-tray thermal transfer
Finally, a PostScript.RTM. tray switching printing. Finally, a PostScript.RTM. tray switching capability that allows inadvertent printing on transparency film does not account for the presence of pre-coat or the expense of wasted thermal transfer ribbon and results in an unacceptable and expensive waste of time and materials.
Therefore, what is needed is a method and apparatus for determining whether to pre-coat and print a particular print medium under predetermined combinations of the above-described print medium and printer related variables.